A method of industrial production of ozone using an electrolysis cell has been generally known. FIG. 5 is a typical sectional view of a central part of a conventional electrolysis cell 101. The electrolysis cell 101 includes a plate-shaped anode (hereinafter referred to as "anodic electrode plate") 103, a plate-shaped cathode (hereinafter referred to as "cathodic electrode plate") 104, and a proton exchange membrane (hereinafter abbreviated to "PEM") 102 sandwitched between the anodic electrode plate 103 and the cathodic electrode plate 104.
The anodic electrode plate 103 is a mesh or porous structure permeable to air and water. A basic material of the anodic electrode plate 103 is titanium. As shown in FIG. 5, at least a surface of the anodic electrode plate 103 facing the PEM 102 is coated with a platinum layer 106. The platinum layer 106 can be formed by a plating process. A catalyst layer 105 containing lead dioxide is interposed between the platinum layer 106 and the PEM 102.
A basic material of the cathodic electrode plate 104 is a stainless steel or titanium. The cathodic electrode plate 104 is a mesh or porous structure permeable to air and water. As shown in FIG. 5, a surface of the cathodic electrode plate 104 facing the PEM 102 is coated with a hydrogen generating catalyst layer 107 containing a metal, such as platinum.
An anode collector plate 108 and a cathode collector plate 109 are attached to the outer surfaces of the electrode plates 103 and 104, respectively, to connect the electrode plates 103 and 104 to a power source. The components laminated between the anode collector plate 108 and the cathode collector plate 109 are compressed to connect the same electrically. The collector plates 108 and the cathode collector plate 109 are provided with a plurality of openings 110 and a plurality of openings 111, respectively. Thus, a plurality of water passages or air passages are formed between the exterior of the electrolysis cell 101 and the PEM 102 through the electrode plates 103 and 104 and the collector plates 108 and 109.
Water is supplied through the plurality of openings 110 to the electrolysis cell 101 and electric current is supplied through the electrode plates 103 and 104 for the electrolysis of water. Then, hydrogen ions migrate from anodic electrode plate 103 through the PEM 102 toward the cathodic electrode plate 104, electrons are supplied to hydrogen ions by the cathodic electrode plate 104 to produce hydrogen. At the same time, oxygen is produced at the anodic electrode plate 103. Part of the oxygen is converted by the catalytic action of the catalyst layer 105 containing lead dioxide into ozone.
As mentioned above, the catalyst layer 105 containing lead dioxide of the electrolysis cell 101 augments ozone generation. Therefore, ozone generating efficiency and the reliability of the electrolysis cell 101 are greatly dependent on the condition of the catalyst layer 105. In the conventional electrolysis cell 101, a lead dioxide film is deposited on the surface of the anodic electrode plate 103 by electrodeposition. The platinum layer 106 is used to improve the adhesion of lead dioxide deposited on the anodic electrode plate 103 by electrodeposition.
The lead dioxide film thus formed is hard and inflexible. Consequently, the lead dioxide film has a drawback that the same is subject to cracking and comes off the anodic electrode plate 103. Therefore it is impossible to mass-produce anodic electrode plates of a desired shape by depositing lead dioxide in a lead dioxide film on a surface of a titanium plate having a large area by electrodeposition and cutting the titanium plate coated with the lead dioxide film in the desired shape because of the foregoing drawback of the lead dioxide film. Therefore, a conventional method makes titanium plates of a desired shape by cutting a large titanium plate and individually forming lead dioxide films on the titanium plates by electrodeposition to make anodic electrode plates, which is unsuitable for mass production and increases the manufacturing cost of the electrolysis cell.
Physical irregularities are liable to be formed in a film formed by electrodeposition, a film formed by electrodeposition is liable to have irregular resistance and it is difficult to form a smooth film of a uniform film quality by electrodeposition. When a lead dioxide film having physical irregularities and a non-uniform film quality is pressed against a PEM, portions having different resistance of the lead dioxide film are pressed in different degrees of contact against the PEM. Consequently, the PEM has portions differing from each other in electric conductivity. Portions having a high resistance generate heat when a current flows therethrough. The heat generated by the PEM reduces ozone generating efficiency and it is possible that the heat damages the PEM. Therefore, conditions for electrodeposition including the temperature and the concentration of an electrodeposition solution are controlled severely and the conditions are adjusted minutely by manual operations to form a uniform film by electrodeposition, which increases the manufacturing cost.
Since the electrodeposition of a lead dioxide film is performed in an oxygen atmosphere, the energy level of the crystal lattice of lead dioxide tends to be dependent on how the conditions were during electrodeposition. The energy level of a lead dioxide film as deposited by electrodeposition falls with time and hence the physical properties of lead dioxide are subject to change with time. It is actually experienced that the ozone generating efficiency of the ozone generating electrolysis cell is dependent on time elapsed after incorporation of the lead dioxide film formed by electrodeposition into the electrolysis cell, (i.e., whether a current is supplied immediately after incorporation of the lead dioxide film into the electrolysis cell, or whether a current is supplied some time after incorporation of the lead dioxide film into the electrolysis cell, electrolysis is either started immediately after the commencement of supplying water to the electrolysis cell or electrolysis is started some time after the commencement of supplying water to the electrolysis cell, respectively) and operating conditions of the electrolysis cell. Such performance of the electrolysis cell is a significant problem in the reliability of the electrolysis cell.
To cope with those problems in the electrodeposited lead dioxide film, a technique disclosed in Japanese Patent Laid-Open No. Hei 8-213027 (Inventor: Kato) employs a flexible catalyst sheet containing lead dioxide. More concretely, a catalyst sheet is formed by filling numerous voids in an oriented, porous polytetrafluoroethylene sheet (PTFE sheet) with a mixture of lead dioxide and a solid electrolytic resin, wherein the PEM is also formed from this resin. The catalyst sheet, i.e., anodic catalyst, a PEM and a cathodic layer, i.e. a carbon paper sheet containing platinum, are superposed in that order to form a superposed structure. The superposed structure is compressed and heated at a temperature in the range of 120 to 140.degree. C. by hot pressing to obtain a laminated structure. The laminated structure is sandwiched between an anodic electrode plate and a cathodic electrode plate to complete an electrolysis cell.
A technique disclosed in Japanese Patent Laid-open No. Hei 11-131276 (Inventor: Mitsuta et al.) spreads lead dioxide powder on a PTFE sheet laminated to a PEM to form a catalyst sheet. An anode collector plate, an anodic electrode plate, the catalyst sheet containing lead dioxide, a PEM, a cathodic electrode plate and a cathode collector plate are superposed in that order to form a superposed structure, and the superposed structure is compressed and heated at 160.degree. C. by hot pressing to fabricate a electrolysis cell.
In the electrolysis cells mentioned in those cited references, i.e., Japanese Patent Laid-open No. Hei 8-213027 (Inventor: Kato) and Japanese Patent Laid-open No. Hei 11-131276 (Inventor: Mitsuta et al.), employing the flexible catalyst sheet containing lead dioxide, problems attributable to the cracking and falling off of the electrodeposited lead dioxide film can be solved and electrolysis cells can be produced at a high efficiency. Since the electrolysis cell is provided with the catalyst sheet having a uniform quality, deterioration of reliability due to irregularity in energy level and physical properties can be avoided. However, lead dioxide is very highly oxidizing and very susceptible to reduction and is also a substance very unstable under heat. The techniques mentioned in the foregoing cited references use a heating process or the hot processing that heats the superposed structure at a temperature in the range of 120 to 160.degree. C. in forming the catalyst sheet containing lead dioxide. Therefore, it is possible that lead dioxide is decomposed during the heating process. If the surface of the catalyst sheet has regions in which the thermal decomposition of lead dioxide has occurred, the surface of the catalyst sheet has irregular resistance distribution and the operation of the electrolysis cell including this catalyst sheet is unstable. Although different in cause from the electrodeposited lead dioxide film, such a catalyst sheet, similarly to the electrodeposited lead dioxide film, deteriorates the reliability of the electrolysis cell.